'I don't believe in God, but I miss Him.' Julian Barnes' new book is, among many things, a family memoir, an exchange with his philosopher brother, a meditation on mortality and the fear of death, a celebration of art, an argument with and about God, and a homage to the French writer Jules Renard. Though he warns us that 'this is not my autobiography', the result is a tour of the mind of one of our most brilliant writers.